I think the next Buono! Festa will likely be in 2020. After all it's one of those events that draw the world's eyes every four years.
I'd be more than happy to be proven wrong with another concert earlier.
Sadly, I am thinking that 2020 is probably too optimistic.
Momo is leaving the entertainment industry entirely for at least 5 years to teach and get more training and experience. After that, she is noncommittal but is not ruling out maybe, somehow, perhaps coming back as a TV Idol, hosting a children's show, and so on in that vein, but for the foreseeable future she will be effectively retired from idolhood and assuming her less-used chara of "Tsugunaga-sensei" out of the public eye.
Airi has not announced any particular post-ºC-ute plans other than a vague promise to stay in the industry and surprising the world + dog with that out-of-the-blue, exciting guest appearance with the always-enjoyable SCANDAL; it is not even clear if she will re-sign with UFP or UFC (like Miya did). But Airi has been and always will be the driving force behind keeping Buono! going; if she takes a fit to have a Buono! reunion concert on her 30th birthday (for example), do not put it past her to cajole Momo back into the biz, at least temporarily. But that is very, very much a longshot for now.
The only certainty I think is that going forward it will be easy for any one of them to shoot Eji a quick note on LINE and get the ball rolling on putting Dolce back together as needed; Eji loves backing Buono! and it shows, so at least that part of the equation should remain constant in the years to come.
(I love Aa! since 2003 so to me Buono is just a variation of Aa! 
I would like them to release another single since they're probably going to disband for good after °C-ute and Momoko's graduations (mostly because of Momo's since she's going to retire from the entertainment industry) but I dunno if management is willing or even considering it since So La Si Do Nee Nee sold pretty low for some reason 
